Recent data provide strong evidence that CXCR4 signaling plays a major role in T-cell leukemia cell maintenance and leukemia initiating activity, and targeting CXCR4 signaling in T-ALL cells reduces tumor growth in an animal model. In this study, the investigators propose that the addition of BL-8040 to nelarabine as a salvage therapy for patients with relapsed/refractory T-ALL/LBL will result in a higher CR rate than nelarabine alone without an increase in toxicity and will allow patients to proceed to a potentially curative allogeneic hematopoietic cell transplant.